Bananabread is the spiritual successor to Syntensity (which added a Javascript scripting layer to Sauerbraten) only this time running inside the player's browser. It was ported using Emscripten, a C++ (LLVM) to Javascript compiler, and seems to have quite a lot of Sauerbraten's functionality ported over, including bots, multiple weapons, map editing, and even a Javascript api to control camera cutscenes.
It currently runs in the development versions of Firefox and Chrome and seems to get about 1/3rd to 1/4th of native fps. Get the source here: https://github.com/kripken/BananaBread